1. Field of the Invention
This invention relates to superconducting ceramics, and in particular to a process utilizing fluidized bed chlorination to extract rare earth values from ore and the preparation of alkoxide precursors for the fabrication of superconducting ceramics.
2. Description of Related Art
The recent discovery by Muller and Bednorz of ceramic superconductors which are superconductive at relatively high temperatures (e.g. now 90 K. or above) has dramatically increased the interest in superconductivity and resulted in a large number of publications. The previous metallic superconductors such as niobium-tin or niobium-titanium required expensive liquid helium cooling. The new ceramic superconductors can be cooled by a relatively inexpensive means, such as liquid nitrogen. The ceramic superconductors have generally been oxides of at least one rare earth, at least one alkaline earth metal, and copper. Typically, the superconductors have been prepared by milling oxides of the rare earth and copper with an alkaline earth metal carbonate (e.g. yttrium oxide, copper oxide, and barium carbonate) and firing the mixture in an oxidizing atmosphere at 1000-1100 C., (typically regrinding, refiring, etc.) and then annealing the ceramic and oxygen at 400-900 C. for an extended period of time (e.g. 0.5-5 days).
Ku et al. in a Paper entitled "Superconductivity and Phase Stability in the Heavy Rare Earth Quaternary Compounds RBa.sub.2 Cu.sub.3 O.sub.7 (R=Ho, Er, Tm, Yb, Lu)" in the Symposium of the MRS spring meeting, 1987, discussed superconductivity in the lanthanum-barium-copper oxide system. Their powders were milled, pressed, sintered, ground, repressed and annealed to prepare samples.
Yoshizaki et al. in a paper entitled "Superconducting Properties of La.sub.1.85 Sr.sub.0.15 CuO.sub.4 Made by Hot Press and Sintered Methods" investigated superconducting transition properties by resistivity and magnetization in lanthanum-strontium-copper oxide for hot pressed and sintered samples. A single crystal was obtained in a portion of one sample.
Christen et al. in a paper entitled "Correlations Among Thermal Processing, Superconducting Properties and Microstructure in La.sub.1.85 Sr.sub.0.15 CuO.sub.4 " synthesized materials beginning with co-precipitation of lanthanum oxide, strontium oxide, and copper oxide from solution in molten urea followed by cold pressing into pellets, sintering/reacting at 1100 C. for four hours in air and annealing in flowing oxygen at 900 C. for 16 hours.
Willis et al. in a letter to the editor entitled "Superconductivity Above 90 K. in Magnetic Rare Earth-Barium-Copper Oxides" (Journal of Magnetism and Magnetic Materials 67, 1987, North Holland, Amsterdam) report measurements of superconducting and magnetic behavior on samples which were prepared by sintering the rare earth oxide, copper oxide, and barium carbonate in an oxygen atmosphere at 1000 C., regrinding and resintering at least twice more to promote reaction and obtain the desired phase.
Shamoto et al. in the Japanese Journal of Applied Physics, April, 1987, article entitled "Effect of Vacuum Annealing on the Superconducting Transition Temperature of La-Sr-Cu-O System" report the effect of vacuum annealing on the superconducting transition temperature of the superconductor lanthanum-strontium-copper oxide system. Their starting materials were apparently oxides pressed and sintered at about 1100 C.
Uwe et al. in the May, 1987 Japanese Journal of Applied Physics paper entitled "Affect of Hetero-Valiant Ion Doping in the High T.sub.C Y-Ba-Cu-O Superconductor" discussed the effect of cerium or lanthanum doping on the resistive transition of high T.sub.C superconductors (i.e. yttrium-barium-copper oxide). The samples were subjected to a procedure in which the material was pulverized, pressed and fired at 850-1000 C. for 2-10 hours in air or oxygen, with the procedure performed two or three times. Some of the samples were then annealed in oxygen at 700 C. for two or three hours. Their doping did not improve the properties and they saw some degradation.
Kasowski et al. in a paper received Mar. 25, 1987 entitled "Electronic Structure of Pure and Doped Orthorombic La.sub.2 CuO.sub.4 " investigate the electronic structure of orthorombic lanthanum-copper oxide and discuss the implications for superconductivity.
Cooke et al. in a paper entitled "Thermally Stimulated Luminescence from rare Earth Doped Barium Copper Oxides"discuss luminescence and emission spectrum measurements of rare-earth-doped barium-copper oxides. The loss of luminescence sensitivity with time, especially when the samples were maintained in vacuum, and the propensity of oxygen-defect perovskites to readily lose or gain oxygen were noted. It was suggested that these measurements might present a very sensitive way to investigate the problem of oxygen stability in these materials.
Braginski et al., in U.S. Pat. Nos. 4,411,959 and 4,575,927 issued Oct. 25, 1983 and Mar. 18, 1986, respectively, teach a submicron particle superconductor arrangement in which brittle superconductive particles remain unsintered in the fabricated wire, thus give a ductile wire, even though the superconducting material is brittle. The small particles provide spacing between particles of much less than the Ginzburg-Landau coherence lengths to avoid any significant degradation to T.sub.c. U.S. Pat. No. 4,419,125 to Charles et al. on Dec. 6, 1983 teaches using liquid alkali metal to co-reduce a mixture of solid halides to produce such submicron powders. These three patents are hereby incorporated by reference.
Naitou et al. in U.S. Pat. No. 4,650,652, issued Mar. 7, 1987, relates to a process for recovering high purity rare earth oxides from a waste rare earth phosphor. The process utilizes dissolving waste rare earth phosphor in an excess amount of acid, adding oxalic acid to obtain precipitates of rare earth oxylates, washing precipitates and baking precipitates.
Ozaki et al. in U.S. Pat. No. 4,507,254, issued Mar. 26, 1985, relates production of a rare earth metal alkoxide by reacting a rare earth metal carboxylate with an alkali metal alkoxide in an inert organic solvent or liquid under anhydrous conditions.
U.S. Pat. No. 4,244,935, issued to Dell on Jan. 13, 1981, relates a method of forming the chloride of a metal-oxygen containing substance based on a fluid coking technique. It should be noted that the commercial process for making zirconium metal utilizes a fluidized bed process in which the ore is subjected to a chlorination step which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated by differential condensation). U.S. Pat. No. 3,895,097, issued to Langenhoff et al. on July 15, 1975, also relates to a process for reacting metal oxides with chlorine.
U.S. Pat. No. 4,670,573, issued to Greco et al. on June 2, 1987, relates to the preparation of metal alkoxides from metals and alcohols. The description of the prior art lists references that relate to the reaction of metals with alcohol to form metal alkoxides. Kirk-Othmer also discusses metal oxides of higher, unsaturated, or branched alcohols made from lower metal alkoxides on page 1, lines 25-50.
U.S. Pat. No. 4,472,510, issued to January on Sept. 18, 1984, relates to a process of making glassy ceramics, including a method of preparing a carbon-containing monolithic glassy ceramic including a metal alkoxide which hydrolyzes and polymerizes in the presence of water.